In Oliver's schematic it is C5. C5 is 25uF in his conversion schematic which connects the cathode to ground and bypasses the cathode resistor. Oliver recommended replacing with a 10uF when using a 5840 and I recommend trying it both ways and comparing while paying particular attention to low-end "depth and imaging" (not necessarily frequency response alone). For my next pair I am currently planning to use Nady 1050 style bodies. I highly recommend using the TAB BV11r's in any case... Especially if you are going with a nice high quality capsule These mic's started out with their T49's which were also very nice but swapped one of the T49's with the BV11r. I'll be swapping the next one very soon. I recommend saving a couple $'s and going with the bv11r's from the get-go.
All I can say regarding the transformers is that I will be using the BV11r's in my next pair. I didn't have any particular issues with the T49's and would recommend them to anyone. Advanced Audio, Peluso, and AMI/TAB all make a more budget priced BV11 or M49 style transformer. Maybe even Cinemag do too or perhaps even Sowter.
I rather enjoy experimenting with different transformers, capsules, parts, and circuit configurations but my experience so far with putting together several DIY mic's and comparing to high-end commercial mic's in critical applications is that Oliver at TAB and Max (ioaudio here on gdiy) both deliver the goods.
The AMI/TAB BV11r transformer along with a quality capsule makes the mic a little more expensive to build but well worth it in my opinion. There are some other budget options too if you want to get it going sooner and don't have funds for the costlier components right away. Plus then when you eventually upgrade the parts you'll have some spares to start another one
As far as a doner-body which is close to the original I don't know that there are any "budget" options but I would also be interested in any economical doner-body options.. The TLM49 lists for about 1k5 so add maybe close to another 1k on top of that for quality parts and you get something that not only sounds the part, but also looks the part.
I would say that it's probably worth it to go with an ac701k and a tlm49 body if you really want something as close to an original as you can get but I don't believe it's absolutely necessary to build a world class mic.
I do a ton of tracking and always need more mic's so my money is better spent on producing more mic's and planning to keep them working for a lifetime.
One positive thing I have to say about using the apex460 style bodies with this circuit (instead of something shorter and fatter like the original) is that the long/slender body lends itself to placement in tighter quarters and I like that.
Cheers,
And best of luck!
Ciao,
jonathan
